Cards containing transponders, such as radio frequency (RF) transponders, are conventionally used to electronically control, detect, and track a variety of items to which the cards are attached. Specifically, such a card (also called xe2x80x9cidentification tagxe2x80x9d), when affixed to or embedded into virtually any object (including livestock and laboratory animals), individually identifies the object using a unique, factory-programmed unalterable code held in a memory mounted on the card and connected to the transponder.
A transponder in such a card can be designed to reflect an incident signal from an interrogation unit (also called xe2x80x9creaderxe2x80x9d) in real time, as described for example, in U.S. Pat. No. 5,479,172 (wherein a xe2x80x9cpower supply voltage has a time varying voltage waveform corresponding to the electric field generated by a reader/controllerxe2x80x9d; see abstract). However, such a real time transponder requires transmission of a large amount of energy because energy of the incident signal and energy of the reflected signal both dissipate at a rate of 1/r2, where r is the distance of the transponder from the reader. Therefore a conventional reader of such identification tags must sense a reflected signal having a strength several orders of magnitude smaller than the strength of the signal sent by the reader.
For another example of such prior art devices, see xe2x80x9cRF Transponder Embedded In Auto Ignition Keys Stymies Car Thievesxe2x80x9d by Milt Leonard, in Electronic Design, Dec. 2, 1993, pages 35-36.
A device (also called xe2x80x9cenergy holding tagxe2x80x9d) in accordance with this invention absorbs and holds (i.e. temporarily stores) energy over a period of time from a first signal, hereinafter xe2x80x9cpower signal,xe2x80x9d and thereafter uses the stored energy to generate a second signal, hereinafter xe2x80x9creply signal.xe2x80x9d The device encodes the reply signal with data from a data source that includes for example a memory or a sensor or both. The circuit in the device includes a power supply that extracts energy from the power signal, and a signal generator that generates the reply signal. The signal generator includes a signal transmitter that encodes data into an electrical signal for transmission in the reply signal.
In one embodiment, the power supply rectifies and stores energy from the power signal over a period of time, and thereafter supplies at least a portion of the stored energy on another line (also called xe2x80x9cpower supply linexe2x80x9d) coupled to the signal transmitter. In one implementation, the power supply includes a rectifier (such as a diode or a p-n junction) having an input terminal (also called power input terminal), and an energy store (such as a capacitor, also called xe2x80x9cenergy storage capacitorxe2x80x9d) coupled to the rectifier thereby to store energy from the power signal, during the period of time (called the xe2x80x9cabsorption periodxe2x80x9d), e.g. 500 ms. The energy storage capacitor has a predetermined capacitance that is selected to provide sufficient power to the signal transmitter for another period (called the xe2x80x9cgeneration periodxe2x80x9d) of time e.g. several microseconds.
The signal transmitter has a reference voltage terminal that is coupled to the power supply. The signal transmitter uses energy drawn from the energy store via the reference voltage terminal during yet another period of time (called the xe2x80x9ctransmission periodxe2x80x9d) to generate a signal that is transmitted as the reply signal. Preferably, but not necessarily, the transmission period is separated from the absorption period by a delay period, e.g. a random delay (such as any period in the range of 0 and 300 ms). The transmission period is at least an order of magnitude smaller than the absorption period so that the reply signal is transmitted at a higher power than the power possible if the two periods are of the same order of magnitude. The one or more (for example 5) orders of magnitude difference between the two periods allows for compression of the stored energy for transmission of the higher power reply signal. Implementation of a random delay allows a number of energy holding tags of the type described herein to be simultaneously activated by an interrogation unit that transmits a power signal just once if each tag transmits in a unique time period. Alternatively, the interrogation unit may transmit a power signal multiple times (e.g. two or three times) to handle collisions caused by simultaneous transmission of a reply signal by two or more of such energy holding tags.
In this particular embodiment, the signal generator also includes, in addition to the signal transmitter, a data source that supplies data to the signal transmitter. The data source has a terminal coupled to a data input line of the signal transmitter. Depending on the implementation, the data source provides on the data input line data from any circuit, such as a non-volatile memory or a sensor (also called environmental sensor) that can sense one or more environmental conditions, for example, temperature, pressure or humidity of the atmosphere surrounding the device. Note that other types of data sources can be used in other embodiments.
In one embodiment, the device is a xe2x80x9cstand alonexe2x80x9d device that communicates with an interrogation unit via wireless signals. In this embodiment, each of the power signal and the reply signal are electromagnetic signals (such as radio frequency signals). Therefore, one variant of the device includes a single antenna that receives as well as transmits the respective power and reply signals, in a time shared manner. In another variant, two distinct antennas are included in the device, one for receipt and the other for transmission. The power supply of the device receives an electrical signal on a line (also called xe2x80x9cantenna linexe2x80x9d) coupled to the antenna when a power signal at a predetermined frequency is incident on the antenna. The signal generator of the device transmits a second electrical signal to the antenna line after a predetermined amount of energy is extracted from the power signal by the power supply, the predetermined amount of energy being at least sufficient to generate the second electrical signal.
In one embodiment, the energy holding tag includes an integrated circuit (abbreviated xe2x80x9cICxe2x80x9d) die that is the sole electronic component in the energy holding tag. The IC die is used as a single (i.e. monolithic) piece free of any electrical connection, e.g. to other discrete electronic components. Such an IC die communicates via electromagnetic signals (such as a radio frequency signal). That is, the energy holding tag of this embodiment does not have various conventional parts, such as bond pads, bond wires, lead frame, or package terminals (such as leads of a pin grid array package or pads of a ball grid array package). In one such energy holding tag, an RF antenna, a power supply, a signal generator and the various conductive lines coupling these parts are all formed as portions of a single integrated circuit (IC) die (also called a xe2x80x9cbare diexe2x80x9d) that is unconnected to any other electronic component. In one implementation of the just-discussed embodiment, the energy holding tag includes an encapsulant, such as a glop of epoxy that covers (partially in one embodiment and completely in another embodiment) the single IC die, thereby to provide protection to the die from environmental factors such as humidity.
In an alternative implementation, the energy holding tag is implemented as an unpackaged die (that does not have bond pads, bond wires, die attach adhesive and glop of epoxy), i.e. a bare IC die without any packaging whatsoever. In another alternative embodiment, the energy holding tag includes an IC die having a connection to an antenna that is formed on a substrate (such as a postage stamp or mailing label) to which the die is attached (e.g. by electrically conductive glue). In such an implementation, the IC die has at least one bond pad for coupling to the antenna (and may have another bond pad for coupling to ground).
Fabrication of an energy holding tag that is devoid of electrical connections (or that has a single connection) is less expensive than fabrication of packaged ICs used in a conventional transponder, due to (1) elimination of conventional integrated circuit package, (2) reduction in die size from elimination of a number of bond pads, and (3) elimination of testing required prior to use of a die packaged in the conventional manner. Moreover, in one embodiment, the transmission period is smaller than the absorption period, and therefore the reply signal is stronger (e.g. up to 100,000 times stronger) than a reflected signal from a prior art transponder of the type described above (in the background section). Furthermore, in one embodiment, the reply signal is also stronger than the power signal.
The signal received by the interrogation unit is thereby increased allowing the energy holding tag to have an antenna sufficiently smaller in size to fit in or on the die in one embodiment.
So, as compared to a prior art reader, an interrogation unit that senses a reply signal from a energy holding tag (as described herein) is less powerful and hence less expensive and provides less interference with other electronic devices that are affected by RF signals. Moreover, as reply signals from energy holding tags can have extremely low power (e.g. in the range of 1xc3x9710xe2x88x9212 to 1xc3x9710xe2x88x9214 W) and yet be detected from a small distance (also called xe2x80x9cdetection rangexe2x80x9d) , such as 1 meter, power signals of the type described herein do not interfere with operation of other electronic devices that may use radio frequency signals, such as domestic microwave ovens, domestic jewelry cleaners, and ultrasonic humidifiers (and so eliminate the need to obtain government permits for operation). In addition, use of interrogation units that detect reply signals only within such a detection range reduces interference from similar or identical energy holding tags in neighboring locations that are outside the detection range of the interrogation unit.